c++ is used for.. A LOT. Stuff like Adobe's photoshop, microsoft office, mozilla firefox, google chrome, internet explorer (some older windows environments, too. like 95 to XP), C++ is a cross-platform language, it works on Mac, Linux and Windows (although linux requires some packeges for it to work).

Java used to be used in applications like printers, faxers, phones, etc. (pre-android/iOS smartphones used java) Android itself uses "java" (reverse engineered java, not actually java but 99.99% the same)
Java is also used a lot for applications, such as webservers, web applets, and rarely even games (minecraft).
As for IDEs, I only know of Eclipse and Intellij, I myself would pick intellij over eclipse any time. (eclipse is poop)

Objective C is not difficult to learn, if you wish to go in the direction of apple things (mac, iOS, etc) then Objective-C is the way to go. What I do recommend is you learn a lot about programming itself first before attempting to make serious things in Objective-C, because it can be really confusing for someone with little to no programming knowledge (Unless you're a genius at it). I myself started out with java (it was a bad idea) and it didn't go well, I ended up learning a lot of PHP, which also made me a better programmer, after which I started doing java again. What I'm trying to say is, try getting better at the easier languages, like php or python before attempting to do more complicated languages like Objective C, java, C#, etc.

 

I mostly used PHP and Java, bukkit plugins are written in Java.

 

Sure, just try to ask more than one question at once, and not spread them out over multiple PMs (Unless it's an urgent question, which I doubt)

 

Objective-C can be easily learned over time with help from youtube and stackoverflow.com plus checking the apple developer manual, just make your first app with thing you know, then add a new feature to every app or update